1. Field of the Invention
This invention relates to lighting devices and systems, and in particular, it relates to light mixing devices having uniform output light distribution.
2. Description of the Related Art
Most lighting apparatus using solid state light source (especially light emitting diodes, LED) emits illumination light by mixing several color lights. For stage lighting red(R), green(G), blue(B) LEDs are turned on respectively or together to obtain monochromatic lights or mixed lights. For example, red and green LEDs can be turned on for mixing yellow light; red and blue LEDs can be turned on for mixing purple light; red, blue, and green LEDs can be turned on by certain power proportion to get white light.
Besides, in some special occasions, such as studios or museums, there is a higher requirement for color rendering index. However, white LEDs alone can't provide light with color rendering index higher than 90, so the light-mixing strategy described above is also generally used to get white light with high color rendering index. For example, U.S. Pat. No. 7,213,940 describes a lighting device utilizing a mixture of white LED and red LED to get white light with high color rendering index.
As is well known, illumination light also has requirement for high optical power. In the high power illumination light device described in Chinese patent publication CN201014341Y, multiple LEDs such as white LED are packaged together in an array and placed in the focus of a Fresnel lens to get parallel light beam. The problem is that its thermal management design is difficult because the LEDs are too close to each other and interfere with each other thermally, which limits the further improvement of power.
To solve the issues above, many solutions have been proposed. A commonly used one is to collimate lights and then mix them directly in the far field. As illustrated in FIG. 1, lights from red LEDs and blue LEDs collimated by different collimators respectively are incident on a screen in the far field and mixed on the screen. This solution is simple, however the uniformity of the mixed light is poor and color shadowing is sometimes produced on the screen. Color shadowing refers to different colors appear on the edge of a shadow cast by an object inserted into the light path. The mechanism of color shadowing is illustrated in FIG. 2. In FIG. 2 section A is illuminated by red light source (labeled R) while the blue light from blue light source (labeled B) is blocked by an object inserting in the light path, so section A appears to be red instead of the color mixed by red and blue. Similarly, corresponding colors will appear in other section of the edge. The reason of color shadowing is that color light beams cannot overlap on the far field screen perfectly due to their different spatial positions. What's more important, for different color LEDs, the collimating angles are different due to the different thickness of LED chips, which can cause color rings in the far field. As shown in FIG. 3, a blue ring will appear on the screen when the collimated blue light has a larger collimating angle.
Therefore, conventional solutions can be used in projection lamp or directional lighting apparatus with low demand for color, but cannot meet the requirement for high color quality.